Definition
The actual ratio of an aircraft's true airspeed to the local speed of sound in the surrounding air. Because the speed of sound varies with air temperature, true Mach number reflects the real Mach value an aircraft is flying at, after correcting the indicated Mach number for temperature and other sensing errors.
Plain English
How fast the aircraft is really moving compared to the speed of sound in the air around it right now, accounting for how cold or warm that air is.
Context Anchor
Seen in instrument flying, high-altitude cruise, aircraft performance, and discussions of Mach limits.
Derivation
Mach comes from Ernst Mach, the physicist who studied how objects behave at and above the speed of sound. 'True' here means corrected and accurate, as opposed to the raw 'indicated' value shown on the instrument.
Why Pilots Care
Determines proximity to critical Mach and risk of compressibility effects such as Mach tuck.
Grounding Statement
A true Mach number of 0.80 means the aircraft is moving at 80 percent of the speed of sound in the air around it.
Intuition Check
“True” does not mean “perfect” here. It means the actual ratio based on the airplane’s real speed through the surrounding air and the speed of sound in that same air.
Example Sentence 1
Above 30,000 feet, the crew monitors true Mach number to stay below the aircraft's maximum operating Mach limit.
Example Sentence 2
The crew monitored true Mach number closely while climbing through the tropopause.